The Combustion Performance of Pelletized Jatropha Seed Residue in

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The Combustion Performance of Pelletized Jatropha Seed Residue in a Vortexing Fluidized-Bed Combustor Pin-Wei Li and Chien-Song Chyang* Department of Chemical Engineering, Chung Yuan Christian University, Chungli District, Taoyuan 32023, Taiwan ROC ABSTRACT: Jatropha curcas is considered a promising energy crop. The response surface methodology is introduced to study the combustion performance of pelletized Jatropha seed residue in a vortexing fluidized-bed combustor, and regression models for the bed temperature (Tb), the combustion fraction (Yb), and the NOx emissions are provided. The result shows that the combustion efficiency is above 99.5%, indicating that JSR pellets burn very well in the combustor. CO can be eliminated by stage combustion accompanied by a controlled freeboard temperature, but NOx increases correspondingly. Tb and Yb have similar trends, showing the interaction between them. The variation in the NOx emissions can be attributed to the shift in volatile-N combustion. In summary, the optimum operating conditions lie slightly above the line of Tb equal to 800 °C.

1. INTRODUCTION With the depletion of fossil fuels and the evolution of greenhouse gases, the demand for renewable energy has risen sharply in recent decades. Biomass is considered a promising energy source due to its carbon-neutral character. In Asia, agricultural waste is the most popular option for this source.1−3 Jatropha curcas, which belongs to the family Euphorbiaceae, is regarded as a potential energy crop.4 Its resistance to drought enables it not to compete with farmland.5 Jatropha is also a versatile plant. Commercial or practical value can be found in its seeds, stems, and even its leaves.6,7 Jatropha seeds are considered one of the best biodiesel feedstocks due to its high oil content with similar properties to fossil diesel.8 However, the Jatropha seed residue (JSR) after oil extraction poses a problem because toxic constituents such as phorbol esters and curcins may induce nausea, diarrhea, and spasms, which precludes its usage as feed.9 Although some detoxification processes have been presented,10,11 their stability and economic feasibility are uncertain. Fortunately, due to its considerable heating value, JSR can be used as a fuel for energy recovery.12 The fluidized-bed combustion technology has been welldeveloped, and it is suitable for biomass combustion because of its high heat transfer and fuel flexibility. Numerous successful studies have verified its utility.13−15 Nevertheless, gas emissions, SO2 and NOx in particular, are major issues for fluidized-bed combustion processes. Because in situ desulphurization can be achieved by limestone addition, NOx is the main gaseous pollutant concern. Fuel-bound nitrogen is believed to be the principal source for NOx emissions in a fluidized-bed combustor.16 On the other hand, operating conditions could also influence the NOx emissions. Higher bed temperatures and excess air, for example, would lead to an increase in NOx emission.17,18 For NOx reduction, stage combustion and flue gas recirculation are widely used strategies,19,20 but in some cases, the results are not satisfactory.21 The combustion fraction, which is an index to determine the main combustion region in a furnace, has been seldom discussed in previous studies, but it is crucial for the arrangement of the heat exchanger surface. The combustion fraction varies with the fuel properties and operating © XXXX American Chemical Society

parameters. Consequently, in a boiler system, the thermal efficiency may be lowered as the fuel or other parameters are modified. The easiest way to quantify the combustion fraction is based on the oxygen consumption,22,23 as will be described in the following section. By estimating the combustion fraction, the main combustion region can be identified to ensure that the fuel is thoroughly utilized. So far, there has been no discussion about the combustion characteristics of JSR in a fluidized-bed combustion system. In this study, the combustion behavior and gas emissions of JSR are statistically investigated in a vortexing fluidized-bed combustor. The in-bed stoichiometric oxygen, excess oxygen, and primary gas flow rate are chosen as variables to study the variation of the bed temperature, combustion fraction in the bed, and gas emissions. The objective is to determine the optimum operating conditions for JSR combustion.

2. METHODS 2.1. Materials. The Jatropha seeds used in this study were purchased from Dawn Exports, India. Because the seed is rich in vegetable oil and can be treated as a feedstock for biodiesel, we extracted the oil after a roasting process. For convenience, the residue after the oil extraction was crushed and pelletized into a cylindrical shape of 6 mm in diameter and 6 mm in length. The Jatropha seed residue (JSR) is prone to become moldy, so the pellets were ovendried and filled with nitrogen for preservation. Table 1 shows the fuel characteristics of the JSR pellets. The proximate analysis was carried out using a thermogravimetric analyzer (PerkinElmer, Pyris 1 TGA), and the heating value was determined by a calorimeter (IKA, C200). The ultimate analysis was committed to authorized organizations to perform. Carbon, hydrogen, nitrogen, and sulfur were analyzed by an Elementar vario EL cube, while oxygen was analyzed by an Elementar vario EL III, and chlorine was measured according to the Taiwan EPA (NIEA M402.00C). The moisture content is rather low due to the drying process described above. The volatile matter content is almost three times that of fixed carbon, which is an indication of easier ignition. In the ultimate analysis, high nitrogen content (4.02%) can be Received: July 9, 2015 Revised: December 16, 2015

A

DOI: 10.1021/acs.energyfuels.5b01552 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels

and the freeboard; the former is used to start up the combustor, and the latter is to maintain the freeboard temperature. The combustor is clad in 25 mm thick ceramic fiber to reduce heat loss through the wall. The combustion gas is divided into two streams, the primary gas and the secondary gas. The primary gas enters the plenum first and then passes through a tuyere-type distributor for even distribution. To generate a gas vortex, the secondary gas was injected tangentially at 0.796 m above the distributor. Both gases are monitored by mass flowmeters (Tokyo Keiso, TF-4130) and heated to 120 °C by preheaters before entering the combustor. The gas is mainly provided from an air compressor; to achieve the desired gas flow rate, a liquefied nitrogen supply system is utilized to compensate for insufficiency. The JSR pellet is fed by a screw feeder, and the feeding point is located 0.5 m above the distributor. To prevent the fuel from undergoing pyrolysis before entering the combustor, we apply a water jacket at the end of the chute. The APCD is mainly used to remove the particulate matter. The fly ash will first be collected by a cyclone; after the flue gas is cooled to approximately 150 °C by a heat exchanger, it passes through a baghouse to capture the finer particles. The flue gas is drawn by an ID fan and eventually discharged to the atmosphere. 2.3. Procedures and Data Acquisition. Before start-up, the silica sand is added into the combustion chamber. The static bed height is fixed at 25 cm throughout the experiments. Afterward, the electric heater of the chamber is switched on to heat the bed material; the sand is stirred every 30 min by the primary gas for uniform heating. Meanwhile, the freeboard temperature is controlled close to the bed temperature over time. When the bed temperature evenly reaches 400 °C, the fuel is fed into the combustor, and the gas flow rates are adjusted to the set values. The freeboard temperature is set at 850 °C, and it requires 20 kW on average to maintain this set value. When the bed temperature deviation is ±5 °C, we consider it to have reached steady state and start to sample. It should be noted that the set value of the freeboard temperature is mainly according to the regulation released from Taiwan EPA.29 Also, the combustor used in this study has a relatively high surface area to volume ratio and there is no refractory lining, so the heat loss through the wall is considerable compared with that of commercial scales. In fact, the freeboard temperature would exceed 850 °C during practical operation of commercial scales, especially for the fuel with high volatile

Table 1. Fuel Characteristics of Pelletized Jatropha Seed Residue fuel

JSR pellet Proximate Analysis (%)

moisture volatile matter fixed carbon ash Ultimate Analysisa (%) carbon hydrogen oxygen nitrogen sulfur chlorine heating value (kcal/kg) lower heating value a

3.95 66.67 23.78 5.60 51.51 7.21 36.91 4.02 0.26 0.09 4524

Dry and ash-free basis.

observed, while the sulfur and chlorine are negligible. The heating value of the JSR is comparable to that of coal, implying its suitability for energy recovery. For all of the experiments, silica sand with 97.5% SiO2 was used as the bed material. The mean diameter is 530 μm, and the apparent density is approximately 2500 kg/m3. The minimum fluidization velocity of bed material is 20.1 cm/s. 2.2. Apparatus. We conducted the experiments in a bench-scale fluidized-bed combustion system, as shown in Figure 1. The system consists of a vortexing fluidized-bed combustor (VFBC), a gas supply system, a screw feeder, and air pollution control devices (APCDs). Details regarding the VFBC design can be found in our previous researches.24−26 The hydrodynamic characteristics and solid flow patterns in the VFBC can be found elsewhere.27,28 The combustion chamber of the VFBC is made of SUS310 with a rectangular cross-sectional area of 0.11 m × 0.22 m and a height of 0.678 m. The freeboard is composed of three 1 m SUS310 tubes and two 0.5 m tubes, and the internal diameter is 0.154 m. Electric heaters with on−off controllers are installed both on the combustion chamber

Figure 1. Schematic diagram of the vortexing fluidized-bed combustion system. B


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Energy & Fuels Table 2. Design Result Based on the Box-Behnken Design run

Sb (%)

Eo (%)

QP (NL/min)

Tb (°C)

O2 (%)

CO (ppm)

NOx (ppm)

η (%)

Yb (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

80 90 100 90 90 90 80 90 100 90 100 90 80 100 90 80

40 30 40 50 40 40 50 50 40 40 30 30 30 50 40 40

200 200 200 200 240 240 240 280 280 240 240 280 240 240 240 280

836.5 870.0 935.8 880.5 795.6 795.4 748.0 697.5 742.0 793.0 853.0 695.0 746.0 861.0 797.0 680.5

4.82 3.19 4.00 5.38 4.22 4.21 5.83 5.32 4.57 4.10 3.35 3.36 3.68 5.47 4.48 5.19

4.4 4.3 4.8 6.6 4.9 4.9 12.8 6.5 7.7 5.7 5.1 8.8 13.7 5.7 5.1 7.3

198.8 104.7 76.6 126.6 186.2 208.3 344.7 606.0 232.2 209.3 62.4 473.9 321.9 118.3 208.1 729.1

99.67 99.80 99.77 99.77 99.64 99.60 99.65 99.66 99.64 99.63 99.69 99.63 99.59 99.66 99.63 99.65

77.9 81.7 81.5 82.3 80.2 79.9 66.6 60.3 80.6 77.9 81.6 58.2 67.8 84.6 81.3 57.9

matter content.30 Hence, it is appropriate to set the freeboard temperature at 850 °C to meet practical situations. The temperature inside the combustor is probed by chromel− alumel thermocouples at 0.07, 0.24, 1.38, 2.43, 3.47, 4.12, and 4.67 m above the distributor, and the bed temperature is the average value of the first three thermocouples (two at 0.07 m and one at 0.24 m). The sampling points of the flue gas are located 0.56, 1.26, 2.26, 3.26, 4.24, and 5.10 m above the distributor. The gas within the combustor is drawn by a vacuum pump and then analyzed by gas analyzers (Horiba, PG-250; Anapol, EU-5000). The gas emissions are calibrated to 6% oxygen on a dry basis. We collect the fly ash at the solid discharge port of the cyclone and analyze the unburned carbon content for the combustion efficiency calculation. In addition, the metal concentrations are also examined. 2.4. Experimental Design. In this study, the feed rate is fixed at 2.45 kg/h, the total input gas is constant at 400 NL/min, and the freeboard temperature is controlled at 850 °C. The main variables are the in-bed stoichiometric oxygen (Sb), primary gas flow rate (QP), and excess oxygen (EO). Sb is defined as the ratio of the actual oxygen flow rate in the primary gas to the theoretical oxygen demand (TOD), and EO is defined as the ratio of excessive oxygen in total oxygen flow rate (QT,O2) to the TOD, i.e., (QT,O2 − TOD)/TOD. We designed the experiment by a statistical method called response surface methodology (RSM), which is commonly used in research and development, and carried out the data analysis using Minitab statistical software. A Box-Behnken design, one type of RSM, is employed in this study. By this design, a three-factorial and three-level experiment can be reduced from 27 to as few as 15 runs, in which three runs are central points to ensure accuracy. The general form of the secondorder model can be written as follows: n

Y=

n−1

n

∑ βi , iXi2 + ∑ ∑ i=1

where QP,O2 = oxygen flow rate in the primary gas, NL/min; Qf,O2 = oxygen flow rate at the outlet of the combustion chamber, NL/min; QT,O2 = total input of oxygen, NL/min; and QF,O2 = oxygen flow rate at the outlet of the combustor, NL/min.

3. RESULTS AND DISCUSSION 3.1. Axial Concentration Profiles of Gaseous Pollutants. Table 2 shows the experimental results of each test run. The values of the gaseous emissions at the exit of the combustor are given. CO and NOx emissions are calibrated to a 6% O2 basis. The SO2 emission is not present because there is no detection at the exit of the combustor. The CO emission is negligible (less than 15 ppm) throughout the runs, indicating that the JSR pellets burned very well in the VFBC. The combustion efficiency is above 99.5%, which is in good agreement with the CO emission. Because the data of the CO emission and the combustion efficiency are almost constant regardless of the operating conditions, it is meaningless and unreliable to analyze them statistically. As a result, only the bed temperature (Tb), combustion fraction (Yb), and NOx emission are discussed in the following sections. For a better understanding of the gas variation within the combustor, the axial gas distribution shall be taken into consideration. Figure 2 shows the axial distribution of the major gaseous pollutants at the central point of the run. The CO concentration exhibits two significant descents along the combustor. The first drop is from approximately 30 000 to 16 010 ppm, and this can be attributed to the secondary gas injection. Upon entering tangentially, the secondary gas stream can effectively enhance the turbulence and extend the residence time, resulting in a greater opportunity for the CO to react with the oxygen. Soon afterward, the CO decreases to less than 30 ppm owing to the freeboard temperature. As mentioned above, the freeboard temperature is set at 850 °C, which benefits the CO oxidation progress. Hence, an extremely low CO emission can be achieved by stage combustion and a controlled freeboard temperature. The SO2 concentration profile, given in Figure 2b, shows a downtrend along the combustor, and 82% of the SO2 is removed before 1.26 m above the distributor. This can be explained by the ash composition and secondary gas. From Table 6, it can be seen that the fly ash is rich in calcium, magnesium, and other metals that favor desulfurization

n

βi , jXiXj +

i=1 j=i+1

∑ βi Xi + β0 i=1

(1)

where Y is the objective function, Xi and Xj are the coded factors, n is the number of factors, and the coefficients, β0, βi, βi,i, and βi,j, are determined by the experimental results. In this case, Sb, QP, and EO are chosen as factors (as shown in Table 2), and four central points are used to promote accuracy, so that there are 16 runs in total. The coded factors, Sb for example, are presented in the form of −1 (80%), 0 (90%), and 1 (100%). To investigate the combustion behavior of the JSR pellets, the bed temperature (Tb), combustion fraction in the bed (Yb), combustion efficiency (η), and gaseous emissions are discussed. The combustion efficiency is calculated by the heat loss method 2. Moreover, Yb can be estimated by the following equation:

Yb = (Q P,O − Q f,O )/(QT,O − Q F,O ) × 100% 2

2

2

2

(2) C


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Table 4. Effect Examinations of the Coded Factors for the Combustion Fraction in the Bed factor

coefficient

SE

T-value

P-value

constant Sb EO QP Sb2 E O2 QP 2 Sb·EO Sb·QP EO·QP

79.86 7.27 0.57 −8.32 −0.42 −4.26 −4.96 1.05 4.78 0.39

1.89 1.33 1.33 1.33 1.89 1.89 1.89 1.89 1.89 1.89

42.31 5.44 0.43 −6.23 −0.22 −2.26 −2.63 0.56 2.53 0.21

0.000 0.002 0.683 0.001 0.830 0.065 0.039 0.598 0.045 0.844

Table 5. Effect Examinations of the Coded Factors for the NOx Emission

Table 3. Effect Examinations of the Coded Factors for the Bed Temperature

a

coefficient

SEa

T-value

P-value

constant Sb EO QP Sb2 E O2 QP2 Sb·EO Sb·QP EO·QP

795.25 47.60 2.87 −88.47 9.85 −3.10 −6.40 1.50 −9.45 −2.00

4.36 3.08 3.08 3.08 4.36 4.36 4.36 4.36 4.36 4.36

182.47 15.45 0.93 −28.71 2.26 −0.71 −1.47 0.34 −2.17 −0.46

0.000 0.000 0.387 0.000 0.065 0.504 0.192 0.742 0.073 0.662

coefficient

SE

T-value

P-value

203.0 −138.1 29.1 191.8 −4.9 13.7 111.1 8.3 −93.7 27.6

16.6 11.7 11.7 11.7 16.6 16.6 16.6 16.6 16.6 16.6

12.23 −11.77 2.48 16.35 −0.29 0.83 6.70 0.50 −5.65 1.66

0.000 0.000 0.048 0.000 0.779 0.440 0.001 0.636 0.001 0.148

of the 16 trials show the similar trend with good reproducibility. The initial decline can be ascribed to the char enrichment and substoichiometric primary gas. A great amount of CO can still be found at 1.26 m above the distributor, implying that some unburned matter (char and volatile matter) is present in this region in spite of the secondary gas injection. The NO reduction is probably accomplished by the following equations:34

Figure 2. Axial distribution of (a) CO, (b) SO2, and (c) NOx at the central run (Sb = 90%, Eo = 40%, and QP = 240 NL/min).

factor

factor constant Sb EO QP Sb2 EO2 QP2 Sb·EO Sb·QP EO·QP

NO + C → 1/2N2 + CO Char

NO + CO ⎯⎯⎯→ 1/2N2 + CO2

(3) (4)

Additionally, because the primary gas enters the combustion chamber with Sb lower than 100%, the global fuel-rich atmosphere may induce a high concentration of NH3, which prefers reacting with NO to form N2 under such conditions.35 However, a recovery of NO is observed between 1.26 and 2.26 m above the distributor. As mentioned above, there is still some unburned matter, including nitrogenous species, and they are prone to oxidize quickly in this zone due to the high freeboard temperature accompanied by excess oxygen. It should be noted that the nitrogen in biomass mainly comes from proteins. The survey done by Makkar and Becker revealed that the crude protein contents of the JSR were high and the amino acid composition was similar to that of soybean meal.36 Further investigation concerning the thermal decomposition of proteins should be made to understand their contributions to NOx emissions. It can be concluded that the higher freeboard temperature favors fuel burnout, but NOx formation will occur as a side effect. Therefore, a compromise should be made to reach the optimal CO and NOx emissions. In summary, the NOx concentration profile shows an unusual trend. Usually, the maximum NOx concentration

Standard error.

reactions,31,32 and the ratios of Ca/S and Mg/S are 0.6 and 2.0, respectively. On the other hand, the gas vortex generated by secondary gas injection can not only retard the upward moving flue gas but cause turbulence, so the residence time of the flue gas could be enhanced. Meanwhile, the fly ash might act with the vortex instead of being carried over, which increased the chances of contact with SO2.33 As a result, SO2 is more likely to react with the metal oxides in the fly ash. Figure 2c shows the NOx concentration profile within the combustor. The NOx concentration first declines and then increases and eventually reaches a roughly steady state at 2.43 m above the distributor. In fact, the NOx concentration profiles D


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Energy & Fuels

Figure 3. Effects of variables on the bed temperature (a) interaction plot and (b) contour plot.

Consequently, the regression model of the bed temperature can be simplified by eliminating insignificant factors as follows:

occurs at the bed surface or at the splashing zone and then decreases along the freeboard due to the NOx destruction reactions. Previous studies provided evidence regardless of the fuel (bagasse, rice husk, soybean, and coal) and the nitrogen content (0.26− 4.39%).37−39 However, this trend, which first declines and then increases and eventually reaches a roughly steady state, was observed in Tarelho et al.’s study in the combustion of bituminous coal with high air staging (lower primary air), and the NO conversion indicated that the first decline was not caused by secondary gas dilution.21 The results validated that the contribution of volatile-N to NO formation cannot be neglected when the fuel has significant volatile matter content. 3.2. Statistical Analysis. To quantify the effects of the variables on Tb, Yb, and NOx emission, we employed the software Minitab for regression analysis. The P-value is the criterion to determine the degree of each factor’s influence; if a P-value is less than 0.05, the influential degree is considered “statistically significant” with 95% confidence. In this study, a 95% confidence interval is chosen for determination. The analytic results of Tb, Yb, and NOx emission are given in Table 3−5, and the adjusted R-squared values are 98.61%, 83.95%, and 96.99%, respectively, showing the good explanatory power of these regression models. From Table 3, it can be seen that Tb is strongly influenced by Sb and QP because their P-values are less than 0.05.

Tb = 795.42 + 47.60S b − 88.47Q P

(5)

where the constant changed from 795.25 to 795.42. Likewise, by P-value determination, the models of Yb and NOx emission can be simplified to the following equations: Yb = 77.52 + 7.27S b − 8.32Q P − 4.96Q P 2 + 4.78S bQ P (6)

NOx = 207.4 − 138.1S b + 29.1EO + 191.8Q P + 111.1Q P 2 − 93.7S bQ P

(7)

The results show that the model for NOx emission exhibits the most complicated structure, while that for bed temperature is a linear combination. The effects on Tb, Yb, and NOx emission will be discussed graphically in the following sections, and the optimal operating conditions will also be presented. 3.3. Effects of Variables. 3.3.1. Bed Temperature. Before the JSR was pelletized, a stable feed rate was hard to reach, leading to a large deviation of the bed temperature (data not shown). It can be attributed to the irregular shape of fuel particles. Similar phenomena were also be found in the combustion of peanut shell23 and sawdust.40 Therefore, the pelletized JSR was used in this study. E


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Energy & Fuels

Figure 4. Effects of variables on the combustion fraction in the bed (a) interaction plot and (b) contour plot.

than that obtained from the combustion of spent activated carbon (100 °C) is much higher F


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Energy & Fuels

Figure 5. Effects of variables on the NOx emissions (a) interaction plot and (b) contour plot.

Table 6. Fly Ash Analysis by ICP-MS (Weight Basis)a

a

run

Na (%)

Mg (%)

Si (%)

P (%)

K (%)

Ca (%)

Fe (%)

Cr (ppm)

Mn (ppm)

Ni (ppm)

Cu (ppm)

Zn (ppm)

Pb (ppm)

11 16

0.428 0.492

7.305 7.377

0.801 1.094

13.71 14.36

13.94 15.52

3.607 4.031

3.252 4.123

171.3 1120

875.7 985.1

87.24 258.3

308.8 291.5

94.48 258.7

74.12 87.31

Note: As and Cd are not detected.

FC ratio, indicating that pelletization is an effective way to retain volatile matter in the bed to burn. The variation of the combustion fraction with the variables is similar to that of the bed temperature. It could be suggested that higher bed temperatures result from a higher combustion fraction in the bed zone, and vice versa. In addition, it is worth mentioning that when the primary gas flow rate is 200 NL/min, the combustion fraction is over 80% even the Sb value is as low as 80%, which implies that lowering the primary gas flow rate can enhance the bed combustion fraction. Such a high combustion fraction in the bed zone for biomass combustion with high volatile matter is beyond our expectations. According to Duan et al.’s study,23 the maximum bed zone combustion fraction of pelletized peanut shell is 56.65%. It can be attributed to the char density and fuel fragmentation. For better heat recovery from the dense phase, the combustion fraction in the bed should be as high as possible. In practical cases such as steam boilers, the combustion fractions may be altered in accordance with the demand of

production lines. Figure 4b can be used as a reference for parameter adjustment. 3.3.3. NOx Emission. Figure 5a shows the effects of the variables on NOx emissions at 6% O2. The results indicate that NOx emissions are strongly affected by operating conditions. As Sb decreases from 100% to 80%, NOx emissions increase noticeably. This could be attributed to the shift of the combustion of nitrogenous volatile matters. With the decrease of the in-bed stoichiometry, more released volatile matter is prone to not burn in the bed but rather enter the lean phase, where they are ignited by the secondary gas as a result of the high freeboard temperature, and NOx derived from volatile-N is consequently formed in the gas phase. Meanwhile, the oxygen content of the secondary gas increases with decreased Sb, which creates a better oxidative atmosphere and favors NOx formation. This is consistent with that the NOx emissions increase with EO. Similar results were observed in previous work during rice husk combustion.44 Based on this discussion, it can be concluded that stage combustion may be counterproductive for NOx reduction if the G


Energy & Fuels

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fuel has high volatile-N content. Therefore, the determination of the volatile-N/char-N content is a top priority when stage combustion is adopted to control the NOx emission. Furthermore, it is of interest that when QP equals 200 NL/ min, the NOx increment is smaller than those with the higher flow rates, as seen in the bottom left of Figure 5a. This behavior is in good agreement with the combustion fraction result. When QP is 200 NL/min, higher combustion fractions originate from the combustion of volatile matter retained in the bed. The NOx generated is then more likely to react with the char and other reductive species, leading to minor NOx variation. Many previous studies support the mechanism of NOx reduction on the char surface.45−48 However, when the primary gas increases, the NOx emissions rise substantially. This could be ascribed to the hydrodynamic changes and less NOx reduction. In this study, the total gas flow rate is fixed. The increase in QP not only enhances the upward driving force but also decreases the vortexing effect of the secondary gas, which thus becomes impotent to retain combustibles in the bed. As a result, the volatile-N after devolatilization is quickly brought into the freeboard to burn. A great amount of NOx is formed at the char-lean freeboard at which NOx reduction is not favorable. Figure 5b gives the NOx variation at different Sb and QP. Because the effectiveness of EO is less, it is fixed at 40% as an average. According to the Taiwan EPA, the NOx emission should be lower than 350 ppm. Therefore, the parameters must be set within the blue area in the contour plot. The fly ash samples are analyzed by ICP-MS (inductively coupled plasma-mass spectrometer) to study if the intrinsic metals have impacts on NOx emissions. The runs with the highest NOx emission (run 16) and the lowest one (run 11) are chosen for the examination. As shown in Table 6, the concentrations of most metals are more or less the same, except for chromium, nickel, and zinc. Although many studies have focused on the catalytic NOx reduction by metal oxides,49−51 there seem to have been no studies on “catalytic NOx formation”. Further investigation should be made to confirm this hypothesis. Based on the discussion of the effects of the variables on Tb, Yb, and NOx emissions, it may be stated that the optimum operating conditions lie slightly above the line when the bed temperature is equal to 800 °C. Under these conditions, defluidization can be avoided, the combustion fraction can be over 70%, and the NOx emissions can be controlled to meet regulations.

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Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSIONS (a) Stage combustion accompanied by a high Tf can effectively eliminate CO but induce NOx recovery owing to the volatile matter combustion. (b) Tb and η indicate that JSR pellets can successfully burn in a VFBC. However, it should be noted that agglomeration may occur if Tb is too high. (c) Pelletization can enhance the Yb for loose fuels with high VM/FC ratio. (d) Lowering Sb is counterproductive for NOx reduction due to the shift in volatile-N combustion. (e) The optimum conditions lie slightly above the line as Tb equals 800 °C. H


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Energy & Fuels

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